This invention relates to electrodeposition of silicon and other metals and to provision of hydride compounds useful in solution as aqueous electrodeposition baths.
Elemental silicon is useful as a coating in a number of different applications. Silicon is a highly effective, corrosion-resistant coating for metals. Silicon coating is used for absorption of incidence solar photons in the generation of electricity by photovoltaic means.
Prior art methods for electrolytic deposition of silicon are difficult and inconvenient in use in that they may require operation at high temperature or in a closed, inert atmosphere. For example, one such method is disclosed in G. Boe, K. Grjotheim, K. Matiasovsky and P. Felner, Electrolytic Deposition of Silicon and of Silicon Alloys, Canadian Metallurigical Quaterly, Volume 10, No. 4 (1971), pp. 281-285. In that method, silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3), were dissolved in cryolite, 3Na.sup.F.AlF 3. The metal is held at approximately 1030.degree. C. and subjected to electrolysis. A copper cathode was utilized and at selected current densities, only silicon was deposited at the cathode. Operation at high temperature using molten materials is required.
Another method for electrolytic deposition of silicon is disclosed in U.S. Pat. No. 3,990,953 to Alfred E. Austin, issued on Nov. 9, 1976. In that method, elemental silicon is deposited on a cathode. A non-aqueous electroplating composition is provided consisting essentially of a non-aqueous solvent solution of anhydrous liquid aprotic dipolar organic solvent such as propylene carbonate containing a silicon solute, such as silicon tetrachloride or trichlorosilane. Deposition on the cathode body is provided through electrolytic reduction of a silicon halide or haloid silane compound. In this method, a non-aqueous electroplating composition must be provided, and the reation is conducted in a sealed vessel in which inert gas must fill the space over the non-aqueous liquid. While operation at relatively low temperatures, e.g. 20.degree.-100.degree. C., is provided, operation in a sealed atmosphere is required.
The present invention comprehends combining silicon in an aqueous solution to provide a plating bath comprising a source of silicon, or by using the same principles to provide a plating bath providing a source of other metals. These compounds will be referred to as hydrides. While there structure is not completely understood at the present time, they do provide the desired electroplating bath. Therefore, the efficacy of the teachings herein should in no way be considered to be affected should reactive species eventually be determined to be other than as described or should a principle be less than fully understood. A repeatable process is provided and explained as fully as possible at the present time.
It is generally believed that provision of a hydride compound suitable as a plating substrate, for example, is impossible to provide. For example, see Douglas M. Considine, Editor, Chemical and Process Technology Encyclopedia (McGraw-Hill, Inc.; New York, N.Y.; 1974), under the description of silicon at page 1031. In describing the chemical properties of silicon, it is stated that "even with diluted bases, Si reacts vigorously, developing H.sub.2 and forming alkali silicates." Silicon hydride is also unexpected in the aqueous form. The entry also states that SiH.sub.4 and the chlorosilanes are instantaneously hydrolyzed with a nucleophilic agent, such as OH.sup.- (water and bases). Silicates do not have utility as a source of metal in an aqueous plating bath. For example, see Ralph K. Iler, The Chemistry of Silica, (John Wiley & Sons; New York, N.Y.; 1979). At page 123, the section entitled "The Nature of Silicate Solutions" discloses a plurality of equilibria between forms of silicate in aqueous solution. None are recognizable as sources of elemental silicon in a plating bath.